By the end of this year, the human population is expected to reach seven billion people, just twelve years after we hit the six billion milestone. But how much more crowded is our planet going to get? Will we keep on expanding indefinitely, or are we approaching the upper limit? The current consensus is that we'll reach our maximum population by around 2050 and then start to slowly decline...but that might be based on two critically flawed assumptions.

The United Nations estimates that the human population will reach nine billion people right around the year 2050. Considering how fast we went from six to seven billion people - and it also took only twelve years to get from five to six billion - it might seem strange that it could take nearly forty years to add another two billion. It's because the birth rate is actually negative in several post-industrial, generally affluent countries in southern and eastern Europe, and this trend is expected to spread outwards.

Indeed, that particular population trend is what scientists are counting on to bring the human population back down to a more sustainable figure. Once the 9 billion mark is reached, the population should max out at about 9.5 billion, and then start to decline as the countries with slowly shrinking populations are more numerous than those that are still growing. In this model, all remaining population growth is pretty much confined to Africa and South Asia.

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But what if that trend doesn't spread across the world? Then humanity might reach the eleven digits, as we could hit the 10,000,000,000 plateau by 2100. Could our planet sustain that many people over the long-term? Well, people have been predicting Earth's fatal overpopulation since the days of Thomas Malthus and our planet has scraped along all right so far, but that's hardly proof we can go on expanding indefinitely, particularly when one adds climate change into the equation.

There's another factor to consider. There's a sharp scientific divide over just how far we can push the human lifespan - some argue it's already pretty much tapped out, while others hold we could get the average life expectancy up to at least 100 by the end of the 21st century. We're hardly talking about immortality, but adding an extra twenty to thirty years of life to all the billions of people could create massive economic and sociological strains.

The current estimates are based on an eventually declining global birth rate and a relatively static life expectancy, both of which keep us beneath 10 billion. But what if both of those are wrong, and we both start living to a hundred and having lots of children? The high-end estimate says we could reach fourteen billion by the end of the century - double what we're looking at right now and almost certainly unsustainable without some massive technological and social leaps.

Now, obviously, a century is still plenty of time to find novel solutions that can sustain an unexpectedly large population. But the fact of the matter is that the best for we can do for the human quality of life and the environmental balance is to actively work to keep populations relatively low, as the Population Council's John Bongaarts explains:

"Almost all of the growth in world population will occur in poor countries, particularly in Africa and South Asia. But if we make much larger investments in family planning right now, the number of people could be closer to 8 billion. Such an investment would have a very beneficial impact on human welfare and any environmental issue we care about."

For now, we can only guess what the future population will be and, in turn, what effect that will have on humanity and our planet. We may well be able to keep the human population to a manageable figure, but as for me...well, I think I'm going to start working on some Caves of Steel. You know, just to be on the safe side.